The present invention relates to hydraulic power transmission systems, and more particularly to a hydraulic drive incorporating a variable displacement pump for powering vehicles and the like.
Hydraulic vehicle drives including torque converters and hydraulicly shifted planetary gear trains are well known. Among the limitations of such drives is the requirement for mechanical shaft/housing continuity between an engine or other power source and driven wheels, which necessitates the use of universal joints and differential gearing, etc. Also, the range of torque multiplication is limited, useable ranges generally necessitating complex torque converter configurations and multiple ratio gear shifting systems that are expensive to provide, the efficiency of the combination being compromised by trade-offs relating to the complexity of the torque converter and the number of selectable gear ratios. For example, under a particular operating condition such as ascending a long grade, one selected ratio might provide insufficient drive wheel torque for maintaining a desired speed, whereas the next available ratio would result in undesirably high engine RPM. A further problem is that the braking systems of vehicles having automatic transmissions have greater wear and shorter life than those having manual transmissions that maintain a selected gear ratio during engine braking.
It is also known to use a positive displacement hydraulic pump that is driven, for example, by a battery powered electric motor, the pump being connected by suitable conduits to hydraulic motors that are coupled to wheels of a vehicle, one advantage being elimination of conventional drive shafts, universal joints, and differential gearing that are components of conventional vehicle drives. The use of electric motor power to the exclusion of internal combustion engines in such drives is dictated by the fixed turns ratio that exists between the positive displacement pump and the hydraulic motors. Actually, the use of such drives is substantially limited to special purpose vehicles such a fork-lifts and the like that operate within a very limited speed range, being preferably battery powered and having other justification for hydraulics, such as for the lift mechanism. Variable displacement hydraulic pumps are also known, but a principle disadvantage is that they are awkward to use in that adjusting the displacement is quite difficult, particularly under load.
Thus there is a need for a variable ratio drive system that overcomes the disadvantages of the prior art.